The present invention relates to electrical machinery and, more particularly, to brushless synchronous electrical generators and motors.
FIGS. 1A, 1B and 1C illustrate the terms used herein to define the geometries of rotary machines and their electrical windings. FIG. 1A shows a right circular cylinder 11, and the corresponding radial, azimuthal, and axial directions. As used herein, a "toroidal" winding is a winding, around a cylinder or torus, that is always perpendicular to the axial direction, and a "poloidal" winding is a winding that is at least partly parallel to the axial direction. FIG. 1B shows a torus 12 partially wound with a toroidal winding 13. FIG. 1C shows a torus 14 partially wound with a poloidal winding 15.
In a conventional synchronous AC electric generator, the rotor winding is connected to a DC current source via rings and brushes. As the rotor is rotated, the magnetic field created by the DC current rotates along with the rotor, inducing an AC electromagnetic force (EMF) in the stator winding. The same design is commonly used for synchronous electric motors: AC current in the stator winding creates a rotating magnetic field that interacts with the rotor's direct magnetic field, causing the rotor armature to rotate.
This design suffers from several inefficiencies. First, the rings and the brushes wear out over time and must be replaced. Second, parts of the stator winding, called "winding ends", protrude beyond the armature. These winding ends do not participate in the generation of electrical current in a generator, or in the generation of torque in a motor; but, unless the windings are made of superconductors, the winding ends contribute to resistance losses. In addition, the associated magnetic fields create eddy currents in electrical conductors outside of the armatures. These eddy currents are an additional drain on the power output of a generator or the power input of a motor.
The reason that rings and brushes are needed in the conventional synchronous machine design is to provide electrical power from a stationary DC current source to a moving rotor winding. There also are brushless designs, one of which, a synchronous induction machine, is illustrated schematically in cross-section in FIG. 2. An axially slotted cylinder 32, made of a ferromagnetic material such as iron, is rigidly mounted on a shaft 30, and rotates within a stationary armature 34. Armature 34 is geometrically in the form of an annulus, with a cylindrical central hole to accommodate slotted cylinder 32, and an interior equatorial slot to accommodate an annular, toroidally wound coil 36. In cross section, armature 34 looks like two opposed U's, as shown. What appear as the arms of the U's are actually two toroidal disks. A set 38 of windings are wound poloidally in slots on the inner periphery of these two disks. Conventionally there are three interleaved windings in set 38, making the synchronous induction machine of FIG. 2 a three-phase machine.
A DC current is supplied to toroidal coil 36, creating a magnetic field around slotted cylinder 32 and windings 38. Because cylinder 32 is slotted and ferromagnetic, as cylinder 32 rotates, the geometry of the magnetic field changes, inducing an AC EMF in poloidal windings 38. Conversely, an AC current introduced to poloidal windings 38 generates a time-varying magnetic field that applies a torque to cylinder 32, causing cylinder 32 to rotate.
The design of FIG. 2 eliminates the need for rings and brushes, but still has the inefficiencies associated with having winding ends that protrude outside the effective zone of electromagnetic induction. In addition, this design is inherently wasteful of space. Coils 36 and 38 must be separated spatially (as shown schematically in FIG. 2) to minimize eddy current losses.
There thus is a widely recognized need for, and it would be highly advantageous to have, an electrical machine (generator or motor) with only stationary windings, arranged geometrically for maximum efficiency.